Blocking contacts for N-type cadmium zinc telluride

ABSTRACT

A process for applying blocking contacts on an n-type CdZnTe specimen includes cleaning the CdZnTe specimen; etching the CdZnTe specimen; chemically surface treating the CdZnTe specimen; and depositing blocking metal on at least one of a cathode surface and an anode surface of the CdZnTe specimen.

ORIGIN OF INVENTION

The invention described herein was made by employees of the UnitedStates Government, and may be manufactured and used by or for theGovernment for governmental purposes without the payment of anyroyalties thereon or therefore.

BACKGROUND OF THE INVENTION

The invention relates in general to x-ray or gamma ray detectors and inparticular to blocking contacts on such detectors.

Cadmium Zinc Telluride (CdZnTe) has gained acceptance as a semiconductordetector material for x-ray and gamma ray applications ranging fromastronomy to medical imaging. One challenge remaining for thistechnology is the production of large volume (greater than 4 cubiccentimeters) detectors free of bulk defects (grain and twin boundaries),which are detrimental to detector performance. CdZnTe can be produced byseveral different Bridgman furnace configurations, for example HighPressure Bridgman (HPB) or Modified Horizontal Bridgman (MHB).

The HPB process produces slightly p-type CdZnTe with a very high bulkresistivity (10¹¹ Ohm-cm) and, therefore, these detectors have lowleakage current noise and good spectral performance. Unfortunately, HPBCdZnTe has a high density of bulk defects that results in a poor yieldand high cost for large volume detectors. The MHB growth processproduces n-type CdZnTe with a much lower density of bulk defects.However, the material has a relatively low bulk resistivity (5×10⁹Ohm-cm) and, therefore, conventional ohmic contacts yield much higherleakage current noise resulting in a poor spectral resolution. To takeadvantage of the lower density of bulk defects in MHB CdZnTe, there is aneed for contacts having low leakage current noise.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a process of making blockingcontacts on n-type CdZnTe.

One aspect of the invention is a process for applying blocking contactson an n-type CdZnTe specimen comprising cleaning the CdZnTe specimen;etching the CdZnTe specimen; chemically surface treating the CdZnTespecimen; and depositing blocking metal on at least one of a cathodesurface and an anode surface of the CdZnTe specimen.

The step of chemically surface treating may include etching in sodiumhypochlorite. The sodium hypochlorite may be about a 1% sodiumhypochlorite solution. The step of etching in sodium hypochlorite may beperformed for about 30 seconds.

The step of chemically surface treating may include rinsing in methanolafter etching in sodium hypochlorite. The step of rinsing in methanolmay be performed for about one minute. The step of chemically surfacetreating may include the step of drying after rinsing in methanol.

The step of depositing blocking metal on at least one of the cathodesurface and the anode surface may include depositing platinum. The stepof depositing blocking metal on at least one of the cathode surface andthe anode surface may include depositing blocking metal at a rate ofabout one angstrom per second. The step of depositing blocking metal onat least one of the cathode surface and the anode surface may includedepositing blocking metal to a total thickness of about 800 angstroms.

The step of etching the CdZnTe specimen may include etching in a brominein methanol solution. The bromine in methanol solution may be about 1%bromine in methanol solution. The step of etching the CdZnTe specimenmay include rinsing the CdZnTe solution in a methanol bath after etchingin the bromine in methanol solution. The step of rinsing may includerinsing the CdZnTe specimen in first and second methanol baths.

The step of cleaning may include agitating the CdZnTe specimen inacetone. The step of cleaning may include agitating the CdZnTe specimenin methanol after agitating in acetone. The step of cleaning may includeagitating the CdZnTe specimen in de-ionized water after agitating inmethanol. The step of cleaning may include examining the CdZnTe specimenunder magnification.

The process may further comprise the step of providing a polished CdZnTespecimen before the step of cleaning the CdZnTe specimen.

Further objects, features and advantages of the invention will becomeapparent from the following detailed description taken in conjunctionwith the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the principal process steps.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention includes a process for applying blocking metal contacts onn-type CdZnTe for the purpose of producing x-ray or gamma ray detectors.The process steps include cleaning with conventional solvents, etchingin a bromine solution to remove cutting and polishing damage, surfacetreating with a chemical, and depositing contacts using a shadow mask incombination with electron beam evaporation of the contact metal.

The inventive blocking contacts reduce CdZnTe detector leakage currentby a factor of 10 at bias voltages typically used for n-type CdZnTedetectors with ohmic contacts (approximately 120 Volts per millimeter ofthickness). The reduced leakage current noise produces improved spectralresolution. In addition, the reduced leakage current noise allows forthe application of higher bias voltages (approximately 200 Volts permillimeter of thickness) thereby promoting full charge collection andfurther improving spectral resolution.

In one embodiment of the invention, the blocking Platinum (Pt) contactsare applied to n-type CdZnTe. The detectors produced in this embodimenthave Pt contacts on both the detector cathode and anode and bothcontacts are produced using the same process. Pt was selected because ofits high work function (5.65 eV) and its demonstrated good adhesion toCdZnTe. The CdZnTe specimens used in the process are supplied withpolished surfaces so that further polishing is not needed. The cathodeand anode patterns are produced using a shadow mask so thatphotolithography is not required.

The process uses two fixtures designed for alignment of the shadow masksrelative to the specimen. There are separate shadow masks for the planarcathode and the pixellated anode. The specimen is aligned within eachfixture. Pre-aligning the specimen in the cathode fixture minimizes thetime the CdZnTe is exposed to the open laboratory environment after thefinal surface chemical treatment and before contact deposition.Likewise, having a separate pre-aligned anode mask fixture minimizes thetime that the CdZnTe is exposed to the open laboratory environmentbetween the cathode and anode deposition process steps.

FIG. 1 is a block diagram showing the principal process steps ofcleaning, etching, surface treating and depositing metal.

CLEANING. For all cleaning steps, the CdZnTe specimen is held in a cleanTeflon basket and only handled using clean Teflon tweezers. The solventbaths can be ultrasonically agitated, or the operator can agitate byhand using the basket. Fresh baths are ideal. Specimen cleanliness iscritical to the process. An exemplary series of cleaning steps are:

-   1. Acetone, five minutes.-   2. Methanol, five minutes.-   3. De-ionized water, five minutes.-   4. Blow dry using dry nitrogen.-   5. Examine specimen under 10 to 20× magnification to insure    cleanliness.-   6. If necessary, repeat cleanings steps 1-4.-   7. Examine specimen under 10 to 20× magnification to insure    cleanliness.-   8. If necessary, use acetone damped swabs to aid in cleaning and    then follow with cleaning steps 1-4.-   9. Examine specimen under 10 to 20× magnification to insure    cleanliness.

ETCHING. The bromine etching solution must be prepared just prior to theetching process. Again, the CdZnTe specimen should be held in a Teflonbasket or holder that allows the etching solution to freely contact theentire cathode and anode surfaces of the specimen. The specimen shouldbe in continuous motion during the entire etching time. An exemplaryseries of etching steps are:

-   1. Etch in 1% Bromine in Methanol solution for four minutes.-   2. Rinse in first Methanol bath, 10 seconds.-   3. Rinse in second Methanol bath, 1 minute.-   4. Blow dry using dry nitrogen.

SURFACE CHEMICAL TREATMENT. The purpose of the surface chemicaltreatment is to restore the surface stoichiometry of the CdZnTe, becausethe bromine etching produces a cadmium rich surface. An exemplary seriesof surface chemical treatment steps are:

-   1. Etch in 1% sodium hypochlorite solution for 30 seconds.-   2. Rinse in Methanol, 1 minute.-   3. Blow dry using dry nitrogen.-   4. Transfer specimen to pre-aligned cathode shadow mask fixture.

CONTACT DEPOSITION. The deposition is performed using an electron beamevaporator. The CdZnTe specimen needs to be loaded into the depositionchamber as quickly as possible after the surface chemical treatment stepto minimize exposure to the laboratory atmosphere. A tungsten crucibleis used for the platinum deposition. Platinum pellets need to bepre-melted in the crucible prior to being used for a contact deposition.Preferably, the platinum crucible is dedicated to this process only.That is, the crucible is not used for other different laboratorydeposition processes thereby helping to insure a repeatable depositionrate at the start of the deposition process. The cathode deposition ispreferably performed first. An exemplary series of contact depositionsteps are:

-   1. Load specimen fixture in evaporator and pump down chamber to less    than 1×10⁻⁶ torr.-   2. Perform cathode platinum deposition at 1 angstrom per second,    total thickness 800 angstroms.-   3. Break vacuum and quickly transfer specimen to pre-aligned anode    fixture.-   4. Load specimen fixture in evaporator and pump down chamber to less    than 1×10⁻⁶ torr.-   5. Perform anode platinum deposition at 1 angstrom per second, total    thickness 800 angstroms.    The deposition of the platinum metal contact can also be done by a    sputtering vacuum deposition process.

One novel feature of the process is the surface chemical treatment thatrestores the surface stoichiometry of the CdZnTe after bromine etching.This processing step dramatically enhances the blocking nature of theplatinum contacts. The improved platinum blocking contacts on n-typeCdZnTe result in an order of magnitude or more reduction in leakagecurrent noise thereby directly improving the spectral resolution. Forexample, a typical CdZnTe pixellated detector suitable for medicalimaging (5 mm thick with a 2.46 mm pixel pitch) having ohmic contactshas a resolution of 7.5% at 60 keV whereas the same exact detectorconfiguration with blocking contacts has a spectral resolution of 4.5%at 60 keV.

An additional benefit of the low leakage current produced by theblocking contacts is compatibility with newer generation applicationspecific integrated circuits being designed for CdZnTe detectorapplications. Many of these new low power ASICs require low leakagecurrent pixels.

While the invention has been described with reference to certainpreferred embodiments, numerous changes, alterations and modificationsto the described embodiments are possible without departing from thespirit and scope of the invention as defined in the appended claims, andequivalents thereof.

1. A process for applying blocking contacts on an n-type CdZnTespecimen, comprising: cleaning the CdZnTe specimen; etching the CdZnTespecimen; chemically surface treating the CdZnTe specimen; anddepositing blocking metal on at least one of a cathode surface and ananode surface of the CdZnTe specimen; wherein the step of chemicallysurface treating includes etching in sodium hypochlorite.
 2. The processof claim 1 wherein the sodium hypochlorite is about a 1% sodiumhypochlorite solution.
 3. The process of claim 1 wherein the step ofetching in sodium hypochlorite is performed for about 30 seconds.
 4. Theprocess of claim 1 wherein the step of chemically surface treatingincludes rinsing in methanol after etching in sodium hypochlorite. 5.The process of clam 4 wherein the step of rinsing in methanol isperformed for about one minute.
 6. The process of claim 4 wherein thestep of chemically surface treating includes the step of drying afterrinsing in methanol.
 7. The process of claim 1 wherein the step ofdepositing blocking metal on at least one of the cathode surface and theanode surface includes depositing platinum.
 8. The process of claim 1wherein the step of depositing blocking metal on at least one of thecathode surface and the anode surface includes depositing blocking metalat a rate of about one angstrom per second.
 9. The process of claim 1wherein the step of depositing blocking metal on at least one of thecathode surface and the anode surface includes depositing blocking metalto a total thickness of about 800 angstroms.
 10. The process of claim 1wherein the step of etching the CdZnTe specimen includes etching in abromine in methanol solution.
 11. The process of claim 10 wherein thebromine in methanol solution is about 1% bromine in methanol solution.12. The process of claim 10 wherein the etching is performed for aboutfour minutes.
 13. The process of claim 10 wherein the step of etchingthe CdZnTe specimen includes rinsing the CdZnTe solution in a methanolbath after etching in the bromine in methanol solution.
 14. The processof claim 13 wherein the step of rinsing includes rinsing the CdZnTespecimen in first and second methanol baths.
 15. The process of claim 1wherein the step of cleaning includes agitating the CdZnTe specimen inacetone.
 16. The process of claim 15 wherein the step of cleaningincludes agitating the CdZnTe specimen in methanol after agitating inacetone.
 17. The process of claim 16 wherein the step of cleaningincludes agitating the CdZnTe specimen in de-ionized water afteragitating in methanol.
 18. The process of claim 15 wherein the step ofcleaning includes examining the CdZnTe specimen under magnification. 19.The process of claim 1 further comprising the step of providing apolished CdZnTe specimen before the step of cleaning the CdZnTespecimen.